Severe Thunderstorm Warning 84, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1005 AM CDT Sun Jul 19 2020

.MESOSCALE DISCUSSION...
1005 AM CDT

Currently, a compact bowing line of storms (and associated MCV)
is moving across Henry, Bureau, Stark, and northern Marshall
counties. The bow is being driven by a strong 80-85kt RIJ seen
nicely on our radar, with reports from NWS Quad Cities confirming
severe winds are reaching the surface. Given the storm is moving
into an airmass into which heavy rain and hence cool air has been
falling over the past few hours, and the low-level shear is not
optimally strong to balance such a strong surging RIJ/cold pool,
confidence in the damaging winds continuing to plow into La Salle
county is not high even though a reservoir of plentiful
instability exists downstream from the cluster. So, while the bow
may continue eastward through northern Illinois, the highest winds
may remain elevated and hence limit the severe weather threat to
an isolated basis. We touched base with SPC and they agree
- though if damage continues into La Salle county, we will have to
entertain the idea of a Severe Thunderstorm Watch through a small
portion of northern Illinois.

Borchardt

&&

.SHORT TERM...
245 AM CDT

Through Today...

Upstream convection has failed to materialize early this morning
across Iowa and and has been quite slow to develop southward
through western Wisconsin. As of 230am CT, convection has reached
only as far south as near La Crosse. The lack of convection has
prevented any development of a surging cold pool that was
originally expected to spread a line of thunderstorms
southeastward into at least the northern CWA prior to daybreak.
While some widely isolated showers have attempted to develop
across southwest Wisconsin and northwest Illinois in association
with an axis of weak sub- 850 hPa WAA, it appears capping with an
EML originally present on the 00Z DVN RAOB and additional warming
around 750 hPa has been enough to prevent convection in the
absence of any other appreciable forcing. Additionally, rather dry
air within the EML up to at least 500 hPa is eroding any weaker
updrafts that develop. At this time, it appears any convection
that develops will be tied to or just ahead of the surface cold
front currently bifurcating Iowa from NE to SW. A sheared out mid-
level wave within weakly cyclonic flow in the vicinity of the
front may provide enough lift to erode some of the capping.

Putting together everything above, have made substantial delays
to the arrival of any potential thunderstorms to after daybreak
for all but the north-central CWA. Coverage will likely be lower
given the lack of any upstream cold pool, so am not expecting
anything more than a loosely broken line of showers and storms
drifting across northern Illinois through mid-morning. The cold
front will gradually become more W to E oriented while shifting
south across the remainder of the CWA, eventually stalling just
south of the CWA by this evening. Scattered thunderstorms will be
possible at any point in the warm and humid airmass ahead of the
front, with the greatest coverage/chance confined to the far
southern CWA late this afternoon. Any thunderstorm today will have
the potential to produce damaging wind gusts given 40 knot flow
as low as 1.5kft this morning and a dry column this afternoon.

A break from the humidity is expected as much drier air arrives
behind the cold front. If no congealed outflow boundaries develop
and surge southeast this morning, the combination of temps rising
to near 90F and dew points still in the mid 70s will allow heat
index values to rise to around 100F for areas south and east of a
Pontiac to Kankakee to Valparaiso line.

Kluber

&&

.LONG TERM...
302 AM CDT

Tonight through Saturday...

Quieter and a bit more pleasant conditions are still expected for
Monday following todays cold front. A surface ridge of high
pressure will shift over the area for Monday bringing with it some
lower humidity and more seasonable temperatures in the mid 80s.
Slight cooler conditions are likely along the lakeshore due to
the likelihood for an afternoon lake breeze. Any shower and
thunderstorm activity on Monday is expected to remain well south
of the area in closer proximity to the surface boundary.

The threat for some showers and thunderstorms will return to the
area late Monday night and especially on Tuesday as the surface
boundary to our south begins to lift back northward across the
area. Several convectively enhanced impulses are likely to drive
bouts of convection in the viscid of the frontal boundary through
midweek. So as this boundary shifts back northward over the area
on Tuesday we will have a decent shot at having a period or two
showers and storms through Midweek.

The chances for convection look to be lower over our area for the
later half of the week and into the weekend as upper level
heights look to build across the central CONUS. The main story
with these building upper level heights will be the potential for
a return to a period of dangerous heat and humidity next weekend.
This potential will need to be monitored.

KJB

&&

.AVIATION...
For the 12Z TAFs...

Scattered thunderstorms continue to develop along a narrow band
from KPIA to KMDW at press time. With each cell, drops in
visibility to 1SM and wind gusts to 30 kt are likely with the
strongest perhaps producing wind gust in excess of 40 kt. With
time, the band should shift south and east. Additional scattered
thunderstorms may develop this morning ahead of a southward-
drifting front (currently inching into far northwestern
Illinois), and along the front itself. The strategy with the
outgoing TAF package is to offer VCTS at most terminals with a
targeted TEMPO with the expected frontal passage along which a
higher coverage of storms is possible. AMDs will be offered if
thunderstorm cells target individual TAF sites.

In the wake of the front, the TAFs become more trivial with
initially gusty but with time relaxing northwest winds and
scattered cloud cover.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

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